Technical Field
The present disclosure relates to an optical system and an image pickup apparatus including the optical system. The optical system is suitable as an image pickup optical system to be used in an image pickup apparatus such as a digital still camera, a digital video camera, a broadcasting camera, a monitoring camera, and a silver-halide film camera.
Description of the Related Art
Among digital still cameras, silver-halide film cameras, and the like, an image pickup optical system to be used in a single-lens reflex camera is required to have a wide angle of view for shooting, which is approximately 40 to 60 degrees. Such image pickup optical system is also required to have a large aperture ratio as well as a capability of focusing an image at a high speed, while maintaining high optical performance.
As a focusing method used in an image pickup optical system, there is a method where the entire image pickup optical system moves, or a method where a lens unit forming a part of an image pickup optical system moves. Among these methods, a method referred to as an inner focus system is known. In the inner focus system, an intermediate lens unit in an image pickup optical system moves to adjust a focus of the lens. The inner focus system enables a reduction of an amount of movement of a lens unit which moves during focusing (a focusing-lens unit), and facilitates miniaturization and weight reduction of the focusing-lens unit, compared to a focusing method where the entire image pickup optical system moves.
As a result, the focusing-lens unit can be moved at a high speed during focusing. Moreover, in such a focusing-lens unit, it is possible to decrease a drive torque at the time of focusing. Therefore, in a case where the focusing-lens unit is applied to an image pickup apparatus having an auto focus detection device, for example, a drive motor can be easily downsized.
Conventionally, there has been known an image pickup optical system having a relatively wide shooting angle of view which employs an inner focus system. U.S. Pat. No. 4,852,984 and Japanese Patent Application Laid-Open No. 1-185507 each discuss an image pickup optical system including, from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, which moves during focusing, and a third lens unit having a positive refractive power, in this order.
When the inner focus system is employed in an image pickup optical system having a large aperture ratio and a wide angle of view, a height of a principal ray of on-axis rays or off-axis rays from an optical axis fluctuates widely, in a lens unit located on an object side relative to a focusing-lens unit. This causes a larger aberration variation in focusing. To address this problem, there has been known an image pickup optical system using a “floating” system, in which a plurality of lens unit moves to adjust a focus. The floating system can reduce an aberration variation caused by bringing a short-distance object into focus.
Japanese Patent Application Laid-Open No. 2012-168456 discusses an image pickup optical system including, from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive or negative refractive power, and a fourth lens unit having a positive refractive power, in this order. In this optical system, the second and fourth lens units move during focusing, thereby high optical performance can be achieved over the entire object distance.